1. Field of the Invention:
This invention relates to a resonant aperture antenna of quasi-planar structure, more particularly to such an antenna which exhibits Gaussian-beam apertured surface power distribution in the microwave-to-submillimeter wave region.
2. Description of the Prior Art:
An antenna for radiating electromagnetic waves into space and receiving electromagnetic waves from space is designed to radiate electromagnetic waves by efficiently transforming oscillating electromagnetic energy into electromagnetic waves which propagate into space through a wave path and to efficiently transform electromagnetic waves propagating through space into energy transmitted through the wave path. In cases where the electromagnetic field radiated by an antenna is considered to be produced as a spatially extending planar electromagnetic field, the antenna is referred to as an aperture surface antenna. The different types of apertured surface antenna include the horn antenna, reflector antenna and lens antenna.
The horn antenna is obtained by gradually flaring the section of a rectangular or circular antenna to the required aperture. The wave front at the aperture is curved and for reducing the deviation from this plane to a small value relative to the wavelength it is necessary to set the opening angle of the horn at an appropriate angle. In addition to being independently usable as an antenna with a gain of about 20 dB, the horn antenna can also be used as the primary radiator of a reflector antenna or a lens antenna. A characteristic of the horn antenna is its good impedance characteristics over a wide frequency range.
The pyramid horn antenna is an antenna obtained by gradually flaring a rectangular waveguide and is excited in the TE.sub.01 mode, which is the fundamental mode of the rectangular waveguide. The TE.sub.01 mode can be considered to appear without modification in the amplitude distribution of the apertured surface and the phase distribution can be determined as the deviation of the wave front. The radiation pattern of the pyramid horn antenna differs between the E plane and the H plane.
The diagonal horn antenna, which has a rectangular aperture, is a horn excited by a wave that is a composite of the TE.sub.01 and TE.sub.10 modes of a rectangular waveguide, and since the distribution in the lateral and longitudinal planes is identical in both modes, an isotropic beam can be obtained.
The conical horn antenna is what is obtained by gradually flaring a circular wave guide and is excited in the TE.sub.11 mode, which is the fundamental mode of a circular waveguide. Since the conical horn is rotationally symmetrical, it is useful in cases where the plane of polarization changes. The amplitude distribution of the apertured surface can be regarded to be the same as that in TE.sub.11 mode and the phase distribution can be determined as a spherical wave whose center is at the apex of the cone.
The rotary parabolic reflector antenna, ordinarily referred to as the parabolic antenna, is an antenna which uses a portion of a rotary parabolic surface as a reflector. This antenna is ordinarily employed as a 30.about.50 dB high-gain antenna and is used in combination with a primary radiator located at the focal point F of the parabolic surface. The reflecting mirror surface functions to transform a spherical wave into a plane wave. A small-aperture pyramid horn, small-aperture conical horn, dipole with reflection plate or the like is used as the primary radiator.
An antenna consisting of two reflecting mirrors, namely, a main reflecting mirror and an auxiliary reflecting mirror, and a primary radiator is referred to as a double reflecting mirror antenna. One that, like the Cassegrain optical telescope, uses a parabolic surface in the main reflecting mirror and a hyperbolic surface in the auxiliary reflecting mirror is referred to as a Cassegrain antenna. A horn antenna is ordinarily used as the primary radiator. One of the two focal points of the auxiliary reflecting mirror is coincident with the focal point of the main reflecting mirror and the other is located to be coincident with the phase center of the primary radiator.
The auxiliary reflecting mirror of the Cassegrain antenna is used as a spherical wave transformer between the primary radiator and the main reflecting mirror. As characteristics of this antenna there can be mentioned, inter alia, that by turning back the electromagnetic beam at the auxiliary reflecting mirror the primary radiator can be positioned near the apex of the main reflecting mirror, whereby (1) the power supply line can be shortened and (2) it is possible by applying mirror surface correction to the two reflecting mirrors to increase the efficiency and reduce the noise of the antenna as a whole, and that owing to the use of the auxiliary reflecting mirror the composite focal distance can be made large, whereby (3) the cross polarization component produced by the reflecting mirror system can be reduced and (4) the range is broad since a primary radiator with a large aperture can be used.
In the parabolic antenna using a rotationally symmetrical parabolic reflecting mirror as its main reflecting mirror, the Cassegrain antenna and the Gregorian antenna, it is necessary to provide a primary radiator, a feed line thereof and an auxiliary reflecting mirror in front of each reflecting mirror. These obstruct the transmission line and are a cause for degradation of the radiation characteristics. As a method for avoiding this, there exist offset antennas, known as the offset parabola antenna, the offset Cassegrain antenna and the offset Gregorian antenna, which use an off-axis parabolic mirror and position the primary radiator or the auxiliary reflecting mirror outside the aperture. These are used for achieving low sidelobe.
Although the various horn antennas have good impedance characteristics over a broad frequency region, technologies have been developed for improvement regarding sidelobe characteristics and axial symmetry. The so-called corrugated horn having a large number of thin fins provided concentrically on the inner wall of a conical horn possesses an axially symmetrical beam and good cross polarization characteristics over a frequency region of about one octave. This horn propagates the EH.sub.11 mode, which is one of the hybrid modes of the corrugated circular waveguide, and when the height of the corrugated waveguide fins is about 1/4 wavelength, the aperture electric field distribution of the EH.sub.11 mode becomes Gaussian distribution-like in the radial direction, thus establishing an axially symmetrical configuration with no variation in the circumferential direction, whereby the directionality of the excited corrugated horn exhibits low sidelobe and little cross polarization component. Owing to its structural complexity, however, a large-aperture corrugated horn is heavy, has many problems in terms of both fabrication technology and cost, and is used only for special purposes. Moreover, in the millimeter wave region where antennas are made small in size, there are difficulties in fabrication technology which make it impracticable at short-millimeter wave and higher frequencies.
On the other hand, thin-film planar circuit technology is expanding from the microwave into the millimeter wave technology region. In cases where an attempt is made to obtain high gain with a planar antenna, array antenna technology is widely used in the microwave region. In multi-element antennas in the millimeter wave-short millimeter wave region above several tens of GHz, however, there is a difficult situation in which practical utilization is not possible because, owing to feed line propagation loss, increasing the number of elements for obtaining sharp directionality leads to a rapid decrease in radiation efficiency.
Many of the prior art antennas described in the foregoing are used mostly in the microwave and lower frequencies and face difficulties in terms of fabrication technology when an attempt is made to use them as millimeter wave-submillimeter wave region antennas. In the millimeter wave and higher frequency range, treatment as a beam becomes important and a new antenna possessing a low sidelobe characteristic and high radiation efficiency is desired. In addition, it is considered that the conventional millimeter wave devices constituted mainly with waveguide technology will be replaced with millimeter monolithic integrated circuits (MMIC) in the near future. For wide and general dissemination of millimeter wave utilization, a need has arisen for the development of a new antenna appropriate for combination with these planar circuit technologies.
With the prior art technologies described in the foregoing it is difficult to achieve sharp directionality and low sidelobe characteristics as well as high antenna radiation efficiency. In particular, at millimeter wave and higher frequencies, there are many cases in which treatment as a quasi-optical beam is advantageous, and in such a case, the efficiency of the antenna for transforming from the guided wave mode to a spatial beam (the radiation efficiency) becomes extremely important. Moreover, the asymmetry in directionality and the sidelobe of the primary radiator used in combination with the reflector antenna are direct causes for degradation of the efficiency and noise characteristics of the whole antenna. On the other hand, a new antenna device is desired for realizing functional millimeter wave utilization technology combined with microwave integrated circuit technology based on recent thin-film device technology.
The present invention was accomplished in the light of the foregoing circumstances and resides in the provision of a new Gaussian-beam antenna usable in the microwave-to-submillimeter wave region, which in addition to possessing high antenna efficiency, high axial symmetry and low sidelobe characteristics and being able to readily achieve a high antenna gain is further suitable for configuring a compact transmitter which has a quasi-planar structure and is combined with thin-film integrated circuit.